User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:96.0) Gecko/20100101 Firefox/96.0

Steps to reproduce:

1. Sign in to any codepen.io account (free)
2. Go to https://codepen.io/habst/pen/zYPqNJr?editors=0010 or any other CodePen editor pen
3. Open the Developer Tools and click the iframe context picker button

Actual results:

On Firefox 96, 5 iframes are shown:

1. https://codepen.io/habst/pen/zYPqNJr?editors=0010
2. about:blank
3. about:blank
4. about:blank
5. about:blank

This does not include the code iframe at cdpn.io, despite the fact that it is displayed on the page and visible in the DOM. So the user can't run code in that execution context without opening the codepen "debug view" which displays the pen preview without any iframes.

Expected results:

On Firefox ESR (91.5.1), 6 iframes are shown:

1. https://codepen.io/habst/pen/zYPqNJr?editors=0010
2. https://cdpn.io/habst/fullpage/zYPqNJr?editors=0010
3. about:blank
4. about:blank
5. about:blank
6. about:blank

This correctly includes the code frame at cdpn.io, and selecting that frame works as expected.

In Firefox 96, when Fission (Firefox's implementation for site isolation) is enabled, iframes from different origins run in different processes.
The iframe picker in version 96 only shows same-process iframes.

This was fixed via Bug 1731740 which is in Firefox 97. Firefox 97 should be released next week, but feel free to try Beta or Developer Edition channels which are already on 97.